Setup will look for an install.wim in the \sources folder. Also it will expect it to be a certain index number (not sure if 1 or 2 but it should be the same as a normal install.wim from an MS Setup ISO.) You may also need to add a description (unsure??).

When you create a custom .wim file, you must include a name and a description, and you must specify the edition by using the /flags option. All values are required by Windows Setup. If a .wim file does not include these values, then the image will not install properly.